You can manually set this very specific requirement:

\documentclass{article}
\usepackage{amsmath,amsthm}
\newtheorem{lemma}{Lemma}
\begin{document}
Some regular text.
\begin{lemma}
This is a lemma.
\end{lemma}
Here is a break of some more text.
\begin{lemma}
\begin{equation}
L_b \circ L_a = L_{L_b(a)} \circ L_b.
\end{equation}
\end{lemma}
Another break of regular text.
\medskip
\refstepcounter{lemma}%
\noindent\leavevmode\rlap{\textbf{Lemma~\thelemma.}}\hfill%
$L_b \circ L_a = L_{L_b(a)} \circ L_b.$\hfill%
\refstepcounter{equation}%
\llap{(\theequation)}\par%
\medskip
Some final text.
\end{document}
\medskip
seemed sufficient in terms of the spacing. However, if need be, one can dig through the code and find the exact vertical spacing above/below the lemma
to make it identical to the rest.
The overlapping (\rlap
and \llap
) is to make sure the equation is eventually centred when using \hfill
.
\documentclass{...}
and ending with\end{document}
.